In the ash dry extraction plant illustrated in EP 0 471 055 B1, the system for recovering fines from the bottom of the extractor and/or of the metallic conveyor placed downstream of the primary extractor is made of scraping chain conveyors indeed positioned onto the bottom, the scraping members thereof, dragged by the conveyor chain, convey the fines which are present onto the bottom of the main conveyor to the discharge of the same.
Systems for cleaning the conveyor bottom are also illustrated in EP 1 409 380, wherein specific buckets hinged to the plates of the metallic conveyor belt, in association with the bottom of the conveyor itself, exert a dragging action of the fines and, in association with a metallic shield placed at the return, they provide the action of recovering fines which are partially brought again by the buckets themselves onto the bearing tract of the main conveyor.
Both said known systems have some drawbacks.
In the first case, the scraping chain conveyor operates wholly dipped into the same atmospherical environment of the belt bearing the ashes. However, the excessive heat transmitted by the transported material makes unadvisable the use of the classical systems for recovering the fines integrated in the same carter of the closed main conveyor.
In the second case, the association of the shield at the return with the recovery members hinged to the plates of the conveyor belt does not avoid the accumulation of fine material at the return itself, by causing a anomalous wear of the conveyor belt which at last drags into the not recovered material. The latter, if highly abrasive, can quickly destroy the integrity of the conveyor belt itself.
Both cases have in common the early wear of the scraping members and of the bottom portions therewith they are constantly in contact.
Another known system for recovering the fine material consists in scraping chains with C-like profile. They are typically installed onto the bottom of the conveyor thereto they are associated and they are characterized by a vertical transportation area placed at the return field of the conveyor itself.
The function is that of recovering the fine in the horizontal tract and to re-convey it onto the loading area of the main conveyor. Said systems, apart from the drawbacks associated to the two cases mentioned above, are characterized by a high wear of the conveyor chain.
In fact, the diameter of the guide wheels and winding rate being equal, the wear extent in the contact points of the catenary meshes is proportional to the number of used guide wheels.
In the catenary with C-like profile, the number of guide wheels not lower than six and the changes in the winding direction on the return guide wheels determine a quick wear of the chain meshes.